Conventionally, power companies supply power using a combination of a base-load power source capable of stably generating power throughout the year, a middle-load power source for generating power to meet a power demand while complementing the base-load power source, and a peak-load power source for generating power to meet a peak power demand in a peak power demand time zone. In this way, the power companies establish a power supply system that can meet the peak power demand in a stable manner.
A power source capable of generating power of a certain amount at all times at low unit cost is assigned as the base-load power source for main power generation. However, during late night hours, when the power demand typically decreases, the base-load power source may cause oversupply of power. As such, power companies, aiming to make effective use of surplus power generated during late night hours and ease the peak power demand during daytime hours, offer rate plans with a lower than normal rate for power used during late night hours and a higher than normal rate for power used in daytime hours to prompt, at low cost, consumers to shift their power demand from daytime to late night hours. As a method to reduce monthly electricity charges utilizing the power generated during late night hours, the use of a stationary storage cell is disclosed. For this reason, there is a demand for introduction of stationary storage cells to consumer facilities such as households and institutions.